1. Field of the Invention
The invention relates to devices for testing electronic circuits, and more particularly, to devices for on-wafer testing of electronic circuits.
2. Description of the Related Art
The large scale production of physically small electronic circuits from semiconductor materials has created a need for economical methods for manufacturing and testing such circuits. Typically, the manufacturing process involves the formation of many discrete electronic circuits from a thin wafer of semiconductor material. After formation of the individual circuits, the wafer is sliced to separate individual circuits so that they can be individually sealed in protective packages for distribution and sale.
Due to the possibility of manufacturing defects which could render individual electronic circuits unacceptable for their intended uses, it is desirable to test each electronic circuit prior to distribution and sale. Furthermore, because the packaging of individual circuits can involve relatively delicate, complex and expensive procedures, it is desirable to test the circuits prior to packaging in order to identify defective circuits and eliminate them from the packaging procedures. Therefore, such electronic circuits often are tested while still integrally attached to the semiconductor wafer on which they were formed.
The advent of increasingly high frequency electronic circuits which can operate at frequencies extending into the gigahertz range has spurred the development of test equipment for on-wafer testing of such high frequency circuits at or near their operating frequencies. The equipment usually includes a probe apparatus for conducting electrical test signals between metallized pads on the wafer adjacent to an electronic circuit being tested and an electronic network for analyzing such signals. A recurrent problem in constructing such a probe apparatus stems from the fact that at very high frequencies, unshielded transmission lines can behave as inductances, blocking electronic signals and thus inhibiting the high frequency testing of electronic circuits.
A first earlier probe apparatus for on-wafer testing of electronic circuits at frequencies in the gigahertz range is illustrated in FIGS. 1 and 2. Referring to FIG. 1, the first earlier probe apparatus (14) comprises a metal base layer (16) overlayed by a dielectric layer (18). An aperture (20) extends through the metal base layer (16) and the dielectric layer (18). Microstrip transmission lines (22) reside upon the dielectric layer (18) and extend to a region adjacent to the aperture (20); whereupon, respective needle probes (24) in electrical contact with respective microstrip transmission lines (22) extend from their respective microstrip transmission lines (22) through the aperture (20) and make electrical contact with an electronic circuit disposed on a semiconductor wafer (26) beneath the first apparatus (14).
FIG. 2 illustrates further aspects of the first earlier apparatus (14). More specifically, there is shown an electrical transition between a center conductor (28) of a coaxial transmission line and the microstrip transmission line (22). It will be understood that signals often are conducted between the first probe apparatus (14) and electric networks (not shown) for analyzing such signals by coaxial transmission lines, and signals are conducted between such coaxial transmission lines and the needle probles (24) by microstrip transmission lines (22). The center conductor (28) is substantially housed within a cylindrical outer conductor shield (30) and is urged against an enlarged region (32) of the microstrip transmission line (22) by a small spring (not shown).
FIG. 3 depicts aspects of a second earlier probe apparatus (34) for conducting electrical test signals between metallized pads (36) adjacent to a wafer-mounted electrical circuit (38) and an electronic network (not shown) for analyzing such signals. The second earlier probe apparatus (34) comprises a plurality of probes (39) formed from a dielectric material. The dielectric probes (39) enclose transmission lines (not shown) which conduct test signals between conductive probe needles (not shown) which depend from the probes (39) and are in electrical contact with the metallic pads (36). The probes (39) both support the transmission lines and the probe needles and provide electrical shielding to the transmision lines. Additionally, grounding probes (42) in electrical contact with certain metallic pads (36) provide electrical grounding during test procedures.
While earlier devices for high frequency on-wafer testing of electronic circuits generally have been successful, there have been limitations on their use. For example, despite the relatively small dimensions of the unshielded needle probes (24) of the first earlier probe apparatus (14), the needle probes (24) still are long enough to experience inductance which can inhibit the accurate testing of electronic circuits which operate above certain relatively high frequencies. Additionally, the geometry of the enlarged region (32), illustrated in FIG. 2, where the center conductor (28) makes electrical contact with the microstrip transmission line (22) may result in poor impedance matching.
Furthermore, although the second earlier apparatus (34) illustrated in FIG. 3 generally features shorter needle probes (not shown) which permit relatively accurate on-wafer testing of electronic circuits even at relatively high frequencies, the probes (39) generally are not easily modified to test different electronic circuits having different numbers and arrangements of metallic pads (36). Additionally, the probes (39) are relatively bulky, and each probe (39) can effectively enclose only a limited number of transmission lines. Thus, for example, where a relatively small electronic circuit surrounded by a relatively large number of metallic pads is to be tested, there may be insufficient space adjacent to such an electronic circuit to bring enough needle probes into electrical contact with the metallic pads to perform accurate testing. These limitations in the use of the second earlier probe apparatus (34) can prove to be drawbacks when, for example, it is desired to perform on-wafer high frequency tests of a variety of integrated electronic circuits having different numbers and arrangements of metallic pads.
Thus, there has been a need for a test probe apparatus for improved on-wafer testing of electronic circuits. The test probe apparatus should be capable of performing accurate testing of high frequency circuits at operational frequencies well into the gigahertz frequency range and should be flexible enough to be easily modified to test a variety of different electronic circuits having relatively large numbers and varied arrangements of metallic pads. The present invention meets this need.